Several types of wastewater collection systems are utilized in commercial and residential environments. These systems include a conventional gravity sewer flow system, a pressurized system which utilizes positive pressure pumps to facilitate sewage flow and vacuum sewer systems.
In a conventional gravity sewer system, the gravitational forces are utilized to induce sewage flow. The structure of a gravity sewer system must be such that the liquid flows from an initial storage tank at a relatively high elevation to a sewage collection area at a lower elevation. The pipeline in a gravity sewer system must have a sufficiently steep slope so that the sewage and water flows therethrough with enough velocity to create a self-cleansing affect. Gravity sewer systems are not cost effective wherein the topography is such that the pipeline cannot be arranged at a sufficiently steep slope to accommodate the required sewage flow.
Positive pressure sewer systems may be used in certain environments wherein gravity sewer systems are not cost effective. Positive pressure sewer systems require the use of one or more pumps located at various wastewater input points so that sewage flow may be maintained by pumping the sewage into a network of relatively small diameter pipelines. Positive pressure systems can also be used in conjunction with a gravity system wherein at least one check valve is arranged at each pump location to serve at the interface between the gravity system which may be utilized at an individual residence and the pressurized system which may be arranged at a remote location, such as under a nearby street.
The third major type of a wastewater treatment system is a vacuum sewer system which may also be referred to as a negative pressure system. In its simplest form, it may include a vacuum collection tank and a vacuum pump located at a collection or pumping station, an initial sewage holding tank, a main pipeline for transporting the sewage from the holding tank to the collection station, and a vacuum valve arranged between the sewage holding tank and the main pipe line. A lateral pipeline which usually has a smaller inner diameter than the main pipeline is arranged between the vacuum valve at the sewage holding tank and the main pipeline.
The vacuum valve may be electrically or pneumatically operated and usually serves as an interface between a conventional gravity plumbing system which may be used to transport sewage to the sewage holding tank and the vacuum portion of the sewer system. Prior art vacuum sewer systems required that a predetermined ratio of wastewater, which may contain both liquid and solid sewage within a water or chemical-based medium, and air be drawn from the sewage holding tank and the outside environment into the main vacuum pipeline. The wastewater and the air were then forced downstream toward the sewage collection station by the pressure differential between the sewage collection tank and the sewage holding tank. The pressure differential exists due to the evacuation of air from the collection tank by using vacuum pumps, such that the collection station end of the main sewage pipeline is at a lower absolute pressure than the atmospheric pressure which normally exists in the sewage holding tank. In other words, the pressure differential creates a hydraulic energy gradient from the sewage holding tank toward the collection station. The hydraulic energy differential drives the wastewater through the open vacuum valve and the connected lateral line and into the main vacuum pipeline towards the collection station.
Operation of vacuum sewer systems is limited by the total pressure differential which may be created between the collection station and the atmospheric pressure at the sewage holding tank. The theoretical upper limit of the pressure differential is between the existing barometric pressure and absolute vacuum. This limit may be quantitatively defined as 760 mm Hg which is approximately equivalent to the pressure exerted by one atmosphere, or 34 feet of water. Practically, this upper limit of the pressure differential cannot be obtained as absolute vacuum is an ideal state. Typical pressure differentials in conventional vacuum sewer systems range between 200-600 mm Hg.
The air that was admitted by the vacuum valve into the associated piping network was necessary to facilitate the flow of wastewater through the system. However, the air took up a certain volume which effectively limited the volume of wastewater which could be drawn into the sewage pipeline at a given time.
In certain prior art systems, an air inlet valve was arranged at remote locations along the main pipeline to facilitate the wastewater flow through problem areas on the pipeline, such as "sags" and "high lift" regions. The sags resulted due to the profile of the associated pipeline which followed the ground surface contour. A sag may result when the pipeline directs the wastewater flow at a downhill angle and then requires the wastewater to flow slightly uphill. A sag may be considered the radius area between the downhill and the uphill slope of the pipeline. The pipeline may retain wastewater in these sags, thus hindering overall wastewater flow. The use of an air inlet valve at a sag region in the pipeline was found to be efficient to force the wastewater which may otherwise be retained in the sag to flow through the pipeline.
With regard to a high lift application, it was found that the use of an air inlet valve at a location along a main pipeline that extended upwardly at a relatively steep angle, helped to facilitate the flow of wastewater through the pipeline at the high lift region.
In conventional vacuum sewer systems, where both air and wastewater are conveyed through the lateral lines, the flow rate of wastewater through the lateral sewage pipeline is typically limited to approximately 15 gallons per minute (GPM). This flow rate may be inadequate for various commercial and residential applications which require handling of large amounts of wastewater. The present invention overcomes the problems associated with inadequate wastewater flow through lateral and main pipelines of a vacuum sewer system.